It is known that sulphur can be decreased in off-gases of the cement industry by spraying in lime-containing sorbents such as, for example, calcium hydroxide. The lime-containing sorbents are sprayed into the off-gas stream, react there with sulphur dioxide and sulphur trioxide of the off-gas to form calcium sulphite and calcium sulphate, which are then incorporated into the kiln meal in the preheater or are separated off at a filter. For this purpose, a defined residence time and sufficient mixing are required. In addition, the off-gas temperature, the off-gas moisture and the concentration of chlorine in the off-gas are critical for high degrees of separation. This is because chlorine has hygroscopic properties and can thereby increase the degree of separation for calcium sulphite and calcium sulphate.
In the context of the production of cement, in addition, sulphur can be incorporated in the raw material in the raw mill. Here also, the incorporation can be increased by an increase in the chlorine concentration.
Introduction of chlorine into off-gases of the cement industry in addition enables separation of mercury to be increased. By oxidation of elemental mercury to mercury chloride, in particular the tendency to adsorption to solid particles, such as filter dust, raw meal or other sorbents, can be increased.
In the off-gases exiting from a (kiln meal) preheater of a cement production plant, generally no chlorine is present, since the chlorine liberated in the clinker kiln at high temperatures is already reincorporated in the preheater downstream of the clinker kiln in the direction of flow of the off-gas. As a result, a chlorine circuit between clinker kiln and preheater is developed which could lead to a continuous increase in the chlorine content. In order to relieve this chlorine circuit, an off-gas bypass is regularly provided in the clinker kiln exhaust, that is to say in the region of the transition between clinker kiln and preheater. Via the off-gas bypass, a substream of the off-gas stream is branched off and then quenched and dedusted at a filter. In this case, the chlorine present is virtually completely incorporated into the bypass dust occurring in the filter. The loaded bypass dust generally is to be ejected from the cement production process on account of the contamination with chlorine and other harmful substances, such as, for example, alkali metals and heavy metals. Treatment of the bypass dust in this case becomes of importance to many operators of cement plants, since, in addition to a loss of material, they give rise to disposal costs.
The bypass dust can be treated in the form of a washing using chemical and mechanical stages. Harmful substances such as, for example, chlorine and heavy metals in this case are removed in various stages from the bypass dust. Processes for treating bypass dust are published, for example, in WO 2012/142638 A1 and WO 2013/113664 A1.